Interpretive Summary: We have shown previously that arginine was the main dietary precursor for the synthesis of the non-protein amino acid citrulline. In this study, we investigated the precursors for citrulline synthesis when mice were fed an arginine free diet. The lack of arginine in the diet did not reduce the synthesis of citrulline. The main precursors were plasma arginine and ornithine. This work shows that, although arginine is needed for citrulline synthesis, a short term arginine deprivation can be overcome by utilizing plasma arginine.

Technical Abstract:
Dietary arginine is the main dietary precursor for citrulline synthesis, but it is not known if other precursors can compensate for when arginine is absent in the diet. To address this question, the contribution of plasma and dietary precursors were determined, utilizing multitracer protocols in conscious mice infused intragastrically either an arginine-sufficient diet [Arg(+)] or an arginine-free diet [Arg(-)]. The plasma entry rate of citrulline and arginine was not different between the two diet groups (156+/-6 and 564+/-30 umol/ (kg/h), respectively); however, the entry rate of ornithine was greater in the mice fed the Arg (+) than the Arg (-) diet (332+/-33 vs. 180+/-16 umol/ (kg/h)). There was a greater utilization of plasma ornithine for the synthesis of citrulline (49+/-4 vs. 36+/-3 umol/ (kg/h), 30+/-3 vs. 24+/-2% of citrulline entry rate) in the mice fed the Arg (-) diet than the Arg (+) diet. There was no difference in the utilization of plasma arginine between the two diet groups for citrulline synthesis, either through plasma ornithine (~29+/-3 umol/ (kg/h)) or at the site of citrulline synthesis (~12+/-3 umol/ (kg/h)). The contribution of dietary proline to the synthesis of citrulline was mainly at the site of citrulline production (17+/-1 umol/ (kg/h)) rather than through plasma ornithine (5+/-0.4 umol/ (kg/h)). Dietary glutamine was utilized only at the site of citrulline synthesis (4+/-0.2 umol/ (kg/h)). Dietary glutamine and proline made a greater contribution to the synthesis of citrulline in mice fed the Arg (-) diet, but remained minor sources for citrulline production. Plasma arginine and ornithine are able to support citrulline synthesis during arginine-free feeding.